Solid polymer cell assembly

ABSTRACT

A cell assembly ( 10 ) includes a first unit cell ( 12 ) and a second unit cell ( 14 ). The first unit cell ( 12 ) and the second unit cell ( 14 ) are juxtaposed such that electrode surfaces of the first unit cell ( 12 ) and electrode surfaces of the second unit cell ( 14 ) are aligned in parallel with each other. An oxygen-containing gas flow passage ( 32 ) includes a first oxygen-containing gas passage ( 38 ) in the first unit cell ( 12 ), an oxygen-containing gas connection passage ( 40 ) in a connection passage member ( 16 ), and a second oxygen-containing gas passage ( 42 ) in the second unit cell ( 14 ). The first oxygen-containing gas passage ( 38 ), the oxygen-containing gas connection passage ( 40 ), and the second oxygen-containing gas passage ( 42 ) are connected serially from the first unit cell ( 12 ) to the second unit cell ( 14 ).

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing ofInternational Application No. PCT/JP2003/008098, filed 26 Jun. 2003,which claims priority to Japan Patent Application No. 2002-186093 filedon 26 Jun. 2002, in Japan. The contents of the aforementionedapplications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a solid polymer cell assembly includinga plurality of unit cells connected together. Each of the unit cells hasan assembly including an anode, a cathode, and a solid polymerelectrolyte membrane interposed between the anode and the cathode. Theunit cells are juxtaposed such that electrode surfaces of the unit cellsare aligned in parallel with each other.

BACKGROUND ART

Generally, a polymer electrolyte fuel cell (PEFC) employs an electrolytemembrane. The electrolyte membrane is a polymer ion exchange membrane(proton ion exchange membrane). The electrolyte membrane is interposedbetween an anode and a cathode to form an assembly (electrolyteelectrode assembly). Each of the anode and the cathode includes basematerial chiefly containing carbon, and an electrode catalyst layer ofnoble metal deposited on the base material. The electrolyte electrodeassembly is sandwiched between separators (bipolar plates) to form aunit cell (unit power generation cell). In use, typically, a pluralityof unit cells are stacked together to form a fuel cell stack.

In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen(hereinafter also referred to as the hydrogen-containing gas) issupplied to the anode. The catalyst of the anode induces a chemicalreaction of the fuel gas to split the hydrogen molecule into hydrogenions and electrons. The hydrogen ions move toward the cathode throughthe electrolyte, and the electrons flow through an external circuit tothe cathode, creating a DC electric current. A gas chiefly containingoxygen (hereinafter also referred to as the oxygen-containing gas) issupplied to the cathode. At the cathode, the hydrogen ions from theanode combine with the electrons and oxygen to produce water.

When the electrolyte membrane of the fuel cell is dried, it is notpossible to maintain the operation at a high output density. Therefore,it is necessary to suitably humidify the electrolyte membrane. For thispurpose, various humidification methods have been adoptedconventionally. For example, in an external humidification method, theelectrolyte membrane of the assembly is humidified by supplying water tothe assembly using a humidifier such as a bubbler provided externally tothe fuel cell. The humidifier humidifies reactant gases (fuelgas/oxygen-containing gas) supplied to the assembly. In an internalhumidification method, a humidifier (humidification structure) forhumidifying the electrolyte membrane is provided in the unit cell.

However, in the external humidification method, since the humidifier isprovided externally to the fuel cell as an additional component, thefuel cell system is large as a whole. Thus, a large space is needed forthe system. In particular, when the load of the fuel cell is increasedrapidly, the humidifier may not have the capability for tracking therapid increase of the load.

In one internal humidification method, strings for absorbing water areembedded in the electrolyte membrane. In another internal humidificationmethod, water from the anode passes through a water permeable plate. Instill another internal humidification method, water absorption stringsare in contact with the electrolyte membrane on the anode side. However,in these methods, when the sufficient level of humidify is not achievedfor some reasons, it is difficult to suitably recover the humidity inthe fuel cell.

DISCLOSURE OF THE INVENTION

The present invention has been made taking the problems into account,and an object of the present invention is to provide a solid polymercell assembly which achieves the desired humidified state reliablywithout using any special humidification devices.

According to the present invention, a cell assembly is formed byjuxtaposing a plurality of unit cells together. Each of the unit cellsincludes an anode, a cathode, and a solid polymer electrolyte membraneinterposed between the anode and the cathode. Electrode surfaces of theunit cells are aligned in parallel with each other. In the cellassembly, at least part of a reactant gas flow passage serially extendsthrough the juxtaposed unit cells. The reactant gas flow passage is apassage of at least one of an oxygen-containing gas and a fuel gas. Themeaning of “at least part of” herein includes at least one of aplurality of reactant gas flow passages, and at least part of a reactantgas flow passage itself.

Since the flow rate of the reactant gas required for reaction in thedownstream unit cell (the unit cell on the downstream side) is takeninto account, and the additional reactant gas is supplied to theupstream unit cell (the unit cell on the upstream side), the flow rateof the reactant gas supplied into the cell assembly is high. Thus, watercondensation in the reactant gas flow passage is prevented, and thehumidity is uniform in each of the unit cells. The current densitydistribution is uniform in each of the unit cells, and thus,concentration overpotential is reduced. Further, simply by increasingthe flow rate of the reactant gas supplied into the cell assembly, waterproduced in each of the unit cells can be discharged efficiently. Watercan be discharged from the cell assembly smoothly.

Moreover, since a long reactant gas flow passage connecting the unitcells are provided. The pressure loss is large. The reactant gas isdistributed smoothly in each of the unit cells, and the reactant gas isdischarged smoothly. In the cell assembly, the unit cells are juxtaposedsuch that electrode surfaces of the unit cells are aligned in parallelwith each other. Thus, the unit cells can be handled independently, andthus, the performance test can be performed individually for each of theunit cells easily and reliably.

As described later in detail, for example, by determining the flowdirections in the oxygen-containing gas flow passage and the fuel gasflow passage (reactant gas flow passages) and the flow direction in thecoolant flow passage to create the humidity difference and thetemperature difference between the upstream unit cell and the downstreamunit cell, it is possible to supply a low humidified gas or anon-humidified gas to the cell assembly. Thus, without using any specialhumidification devices, it is possible to achieve the desired humidifiedstate reliably.

The reactant gas flow passage serially extends through a passage on theupper side of an assembly of the upstream unit cell (unit cell providedon the upstream side in the flow direction of the reactant gas) and apassage provided on the lower side of an assembly of the downstream unitcell (unit cell provided on the downstream side in the flow direction ofthe reactant gas). Thus, water produced in the upstream unit cell isreliably discharged into the downstream unit cell by the gravity. Withthe simple structure, it is possible to prevent the condensed water frombeing trapped in the assembly. The excessive water is efficientlydischarged into the reactant gas flow passage provided on the lower sideof the assembly by the gravity.

The oxygen-containing gas and the fuel gas flow in a counterflow mannerin the oxygen-containing gas flow passage and the fuel gas flow passageas the reactant gas flow passages along the surfaces of the assembly ofthe unit cell. Thus, water moves between the fuel gas flowing throughthe fuel gas flow passage and the oxygen-containing gas flowing throughthe oxygen-containing gas flow passage through the solid polymerelectrolyte membrane. Accordingly, it is possible to reliably preventthe solid polymer electrolyte membrane from being dried. Thus, the lowhumidified reactant gas or non-humidified reactant gas can be suppliedto the cell assembly.

In the structure, a coolant flow passage is provided such that a coolantflows serially from the upstream unit cell provided on the upstream sidein the flow direction of the oxygen-containing gas (hereinafter alsoreferred to as the O₂ upstream unit cell) to the downstream unit cellprovided on the downstream side in the flow direction of theoxygen-containing gas (hereinafter also referred to as the O₂ downstreamunit cell). Thus, temperature of the O₂ downstream unit cell is kepthigher than temperature of the O₂ upstream unit cell.

The O₂ upstream unit cell is a low temperature unit cell and the O₂downstream unit cell is a high temperature unit cell. The lowtemperature unit cell includes the inlet side of the oxygen-containinggas where the humidity is low and the outlet side of the fuel gas wherethe humidity is high. The high temperature unit cell includes the outletside of the oxygen-containing gas where the humidity is high, and theinlet side of the fuel gas where the humidity is low. The humidity inthe O₂ upstream unit cell is high due to the water produced in powergeneration. However, the relative humidity of the oxygen-containing gasis low since the temperature of the O₂ upstream unit cell is high.Accordingly, water condensation does not occur in the O₂ upstream unitcell. The current density distribution is uniform, and the concentrationoverpotential can be reduced.

The structure of the upstream unit cell (low temperature unit cell) isdifferent from the structure of the downstream unit cell (hightemperature unit cell). Optimum structure can be adopted for reaction ineach of the unit cells. Specifically, the assembly of the upstream unitcell and the assembly of the downstream unit cell have the same powergeneration performance when the assembly of the upstream unit cell isoperated at a lower temperature in comparison with the assembly of thedownstream unit cell.

Further, the assembly of the O₂ upstream unit cell has the cathodeincluding a hydrophobic diffusion layer having low porosity, and theanode including a hydrophilic diffusion layer having high porosity. Thehydrophobic diffusion layer having low porosity is provided on the upperside, and the hydrophilic diffusion layer having high porosity isprovided on the lower side.

Thus, when the oxygen-containing gas flows through the upper portion ofthe assembly of the upstream unit cell, in the presence of thehydrophobic diffusion layer having low porosity, the water produced inthe power generation does not move downwardly by the gravity. Therefore,the desired humidity of the oxygen-containing gas is maintainedsuitably. When the fuel gas flows through the lower portion of theassembly of the O₂ upstream unit cell through the O₂ downstream unitcell, the condensed water moves through the hydrophilic diffusion layerhaving high porosity toward the solid polymer electrolyte membrane.Thus, humidity in the surfaces of the solid polymer electrolyte membraneand the electrodes are kept at the optimum level for power generation.Thus, the low humidified oxygen-containing gas or non-humidified gas canbe supplied to the cell assembly.

Further, the assembly of the O₂ downstream unit cell has the anodeincluding a hydrophobic diffusion layer having low porosity, and thecathode including a hydrophilic diffusion layer having high porosity.The hydrophobic diffusion layer having the low porosity is provided onthe upper side, and the hydrophilic diffusion layer having high porosityis provided on the lower side.

Thus, when the fuel gas flows through the upper portion of the assemblyof the downstream unit cell, in the presence of the hydrophobicdiffusion layer having low porosity, the water produced in the powergeneration does not move downwardly by the gravity. Therefore, thedesired humidity of the fuel gas is maintained suitably. Theoxygen-containing gas is humidified at the time of passing through theupstream unit cell. After passing through the upstream unit cell, thehumidified oxygen-containing gas flows through the lower portion of theassembly of the downstream unit cell. The condensed water move throughthe hydrophilic diffusion layer having high porosity toward the solidpolymer electrolyte membrane. Thus, humidity in the surfaces of thesolid polymer electrolyte membrane and the electrodes are kept at theoptimum level for power generation. Thus, low humidifiedoxygen-containing gas or non-humidified gas can be supplied to the cellassembly. Further, the excessive water from the assembly is dischargedefficiently by the gravity into the oxygen-containing gas flow passageprovided at the lower portion of the assembly.

A connection passage member is provided between the juxtaposed unitcells. The connection passage member has a reactant gas connectionpassage and a coolant connection passage for serially supplying thereactant gas and the coolant. Thus, the cell assembly is compact as awhole, and the compact cell assembly can be installed at variouspositions easily and suitably.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing main components of a solidpolymer cell assembly according to a first embodiment of the presentinvention;

FIG. 2 is a view schematically showing distinctive structures of thecell assembly;

FIG. 3 is a view showing change in humidity in first and second unitcells;

FIG. 4 is a view showing change in temperature in the first and secondcell unit cells; and

FIG. 5 is a view schematically showing main components of a solidpolymer cell assembly according to a second embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a view schematically showing main components of a solidpolymer cell assembly 10 according to a first embodiment of the presentinvention.

The cell assembly 10 includes a plurality of unit cells, e.g., a firstunit cell 12 and a second unit cell 14 which are juxtaposed such thatelectrode surfaces of the first and second unit cells 12, 14 are alignedin parallel with each other. A connection passage member 16 is providedbetween the first and second unit cells 12, 14. The first unit cell 12is provided on the upstream side in a flow direction of anoxygen-containing gas (reactant gas) indicated by an arrow A, and thesecond unit cell 14 is provided on the downstream side in the flowdirection of the oxygen-containing gas.

The first unit cell 12 includes a first assembly 18, and the second unitcell 14 includes a second assembly 20. Each of the first assembly 18 andthe second assembly 20 comprises a cathode 24 a, 24 b, an anode 26 a, 26b, and a solid polymer electrolyte membrane 22 a, 22 b interposedbetween the cathode 24 a, 24 b and the anode 26 a, 26 b. Each of thesolid polymer electrolyte membranes 22 a, 22 b is formed by impregnatinga thin membrane of perfluorosulfonic acid with water, for example. Thesolid polymer electrolyte membrane 22 a is a relatively low temperatureelectrolyte membrane, and the solid polymer electrolyte membrane 22 b isa relatively high temperature electrolyte membrane. Namely, the solidpolymer electrolyte membrane 22 a and the solid polymer electrolytemembrane 22 b have the same power generation performance when the solidpolymer electrolyte membrane 22 a is operated at a low temperature incomparison with the solid polymer electrolyte membrane 22 b.

Each of the cathodes 24 a, 24 b, and the anodes 26 a, 26 b includes basematerial chiefly containing carbon, and an electrode catalyst layer ofnoble metal deposited on the base material. A gas diffusion layer(porous layer) such as a porous carbon paper is provided on the surfaceof the electrode catalyst layer.

The cathode 24 a of the first assembly 18 has a hydrophobic diffusionlayer having low porosity. The cathode 24 a is provided on the upperside in the direction indicated by an arrow C1. The anode 26 a of thefirst assembly 18 has a hydrophilic diffusion layer having highporosity. The anode 26 a is provided on the lower side in the directionindicated by an arrow C2. The anode 26 b of the second assembly 20 has ahydrophobic diffusion layer having low porosity. The anode 26 b isprovided on the upper side in the direction indicated by the arrow C1.The cathode 24 b of the second assembly 20 has a hydrophilic diffusionlayer having high porosity. The cathode 24 b is provided on the lowerside indicated by the arrow C2.

The first separator 28 a faces the cathode 24 a of the first assembly18, and the first separator 28 b faces the cathode 24 b of the secondassembly 20. The second separator 30 a faces the anode 26 a of the firstassembly 18, and the second separator 30 b faces the anode 26 b of thesecond assembly 20.

The cell assembly 10 includes the juxtaposed first and second unit cells12, 14, and has an oxygen-containing gas flow passage (reactant gas flowpassage) 32 for supplying the oxygen-containing gas serially from thefirst unit cell 12 to the second unit cell 14, and a fuel gas flowpassage (reactant gas flow passage) 34 for supplying a fuel gas(reactant gas) serially from the second unit cell 14 to the first unitcell 12. Further, the cell assembly 10 has a coolant flow passage 36 forsupplying a coolant serially from the first unit cell 12 to the secondunit cell 14.

In the first unit cell 12, a first oxygen-containing gas passage 38extends between the cathode 24 a of the first assembly 18 and the firstseparator 28 a in the direction indicated by the arrow A. The firstoxygen-containing gas passage 38 is connected to an oxygen-containinggas connection passage 40 formed in a connection passage member 16. Theoxygen-containing gas connection passage 40 is connected to a secondoxygen-containing gas passage 42 formed between the cathode 24 b of thesecond assembly 20 and the first separator 28 b in the second unit cell14.

The first oxygen-containing gas passage 38, the oxygen-containing gasconnection passage 40, and the second oxygen-containing gas passage 42of the oxygen-containing gas flow passage 32 are connected serially suchthat the oxygen-containing gas flows from the first unit cell 12 to thesecond unit cell 14.

In the second unit cell 14, a first fuel gas passage 44 is formedbetween the anode 26 b of the second assembly 20 and the secondseparator 30 b. The first fuel gas passage 44 is connected to a fuel gasconnection passage 46 formed in the connection passage member 16. Thefuel gas connection passage 46 is connected to a second fuel gas passage48 formed between the anode 26 a of the first assembly 18 and the secondseparator 30 a in the first unit cell 12.

The first and second fuel gas passages 44, 48 have a counterflowarrangement with respect to the second and first oxygen-containing gaspassages 42, 38 along the surfaces of the second and first assemblies20, 18. The fuel gas flows in the fuel gas flow passage 34 in thedirection opposite to the oxygen-containing gas flowing through theoxygen-containing gas flow passage 32. The first fuel gas passage 44,the fuel gas connection passage 46, and the second fuel gas passage 48are connected serially such that the fuel gas flows from the second unitcell 14 to the first unit cell 12.

A first coolant passage 50 is formed on the second separator 30 a of thefirst unit cell 12. The first coolant passage 50 has a counterflowarrangement with respect to the second fuel gas passage 48 such that thecoolant flows in the first coolant passage 50 in a direction opposite tothe flow direction of the fuel gas flowing through the second fuel gaspassage 48. The first coolant passage 50 is connected to a coolantconnection passage 52 formed in the connection passage member 16. Thecoolant connection passage 52 is connected to a second coolant passage54 in the second unit cell 14. The second coolant passage 54 has aparallel flow arrangement with respect to the second oxygen-containinggas passage 42 on the second separator 28 b of the second unit cell 14such that the coolant flows through the second coolant passage 54 inparallel with the oxygen-containing gas flowing through the secondoxygen-containing gas passage 42.

The coolant flow passage 36 has a parallel arrangement with respect tothe oxygen-containing gas flow passage 32. The first coolant passage 50,the coolant connection passage 52, and the second coolant passage 54 areconnected serially such that the coolant flows from the first unit cell12 to the second unit cell 14.

Operation of the cell assembly 10 will be described below.

An oxidizing gas such as an oxygen-containing gas is supplied to theoxygen-containing gas flow passage 32, and a fuel gas such as ahydrogen-containing gas is supplied to the fuel gas flow passage 34.Further, a coolant such as pure water, an ethylene glycol or an oil issupplied to the coolant flow passage 36.

The oxygen-containing gas is supplied into the first oxygen-containinggas passage 38 of the first unit cell 12. Then, the oxygen-containinggas flows along the cathode 24 a of the first assembly 18 in thedirection indicated by the arrow A. After the oxygen-containing gasflows out of the first oxygen-containing gas passage 38, theoxygen-containing gas is supplied to the oxygen-containing gasconnection passage 40, and flows in the direction of gravity indicatedby the arrow C2. Then, the oxygen-containing gas flows into the secondoxygen-containing gas passage 42 of the second unit cell 14. Theoxygen-containing gas flows along the cathode 24 b of the secondassembly 20 of the second unit cell 14 in the direction indicated by thearrow A, and is discharged from the second unit cell 14.

The fuel gas is supplied into the first fuel gas passage 44 of thesecond unit cell 14. Then, the fuel gas flows along the anode 26 b ofthe second assembly 20 in the direction indicated by the arrow B(opposite to the direction indicated by the arrow A). After the fuel gasflows out of the first fuel gas passage 44, the fuel gas is supplied tothe fuel gas connection passage 46, and flows in the direction ofgravity indicated by the arrow C2. Then, the fuel gas flows into thesecond fuel gas passage 48 of the first unit cell 12. The fuel gas flowsalong the anode 26 b of the first assembly 18 of the first unit cell 12in the direction indicated by the arrow A, and is discharged from thefirst unit cell 12.

In the first and second assemblies 18, 20, the oxygen-containing gassupplied to the cathodes 24 a, 24 b, and the fuel gas supplied to theanodes 26 a, 26 b are consumed in the electrochemical reactions atcatalyst layers of the cathodes 24 a, 24 b and the anodes 26 a, 26 b forgenerating electricity.

The coolant supplied to the coolant flow passage 36 flows into the firstcoolant passage 50 of the first unit cell 12, and flows in the directionindicated by the arrow A. The coolant flows into the second coolantpassage 54 of the second unit cell 14 through the coolant connectionpassage 52 of the connection passage member 16. After the coolant isused for cooling the first and second assemblies 18, 20, the coolant isdischarged from the second unit cell 14.

FIG. 2 is a view schematically showing distinctive structures of thecell assembly 10 according to the first embodiment of the presentinvention. Specifically, a low humidified oxygen-containing gas(oxygen-containing gas which is humidified to a small extent) or anon-humidified oxygen-containing gas is supplied to the firstoxygen-containing gas passage 38 of the first unit cell 12, and a lowhumidified fuel gas (fuel gas which is humidified to a small extent) ornon-humidified fuel gas is supplied to the first fuel gas passage 44 ofthe second unit cell 14.

After the oxygen-containing gas passes through the firstoxygen-containing gas passage 38 provided on the upper side of the firstassembly 18, the oxygen-containing gas flows through the connectionpassage member 16 in the direction of gravity. Then, theoxygen-containing gas flows into the second oxygen-containing gaspassage 42 provided on the lower side of the second assembly 20 of thesecond unit cell 14.

After the fuel gas passes through the first fuel gas passage 44 providedon the upper side of the second assembly 20 of the second unit cell 14,the fuel gas flow through the connection passage member 16 in thedirection of gravity. Then, the fuel gas flows into the second fuel gaspassage 48 provided on the lower side of the first assembly 18 of thefirst unit cell 12.

The oxygen-containing gas and the fuel gas flow along both surfaces ofthe first and the second assemblies 18, 20 in the opposite directions ina counterflow manner. The coolant and the oxygen-containing gas flow inthe same direction, i.e., the coolant flows from the first coolantpassage 50 of the first unit cell 12 to the second coolant passage 54 ofthe second unit cell 14 through the connection passage member 16 in thedirection indicated by the arrow A.

Thus, the temperature of the first unit cell 12 is lower than thetemperature of the second unit cell 14. Taking the temperaturedifference into account, the solid polymer electrolyte membrane 22 aused in the first assembly 18 is capable of achieving the powergeneration performance equal to the power generation performance of thesolid polymer electrolyte membrane 22 b used in the second assembly 20when the solid polymer electrolyte membrane 22 a is operated at a lowtemperature in comparison with the solid polymer electrolyte membrane 22b.

The low humidified oxygen-containing gas or non-humidifiedoxygen-containing gas is supplied to the cathode 24 a of the firstassembly 18. In order to keep the humidity of the first assembly 18, thecathode 24 a has the hydrophobic diffusion layer having low porosity.The fuel gas flows through the second unit cell 14 before the fuel gasis supplied to the anode 26 a of the first assembly 18. Thus, thehydrogen partial pressure of the fuel gas supplied to the anode 26 a issmall, and the relative humidity of the fuel gas supplied to the anode26 a is high. Therefore, the anode 26 a has the hydrophilic diffusionlayer having high porosity so that water can move toward the cathode 24a smoothly.

Likewise, the low humidified fuel gas or non-humidified fuel gas issupplied to the anode 26 b of the second assembly 20. Thus, in order tokeep the humidity of the second assembly 20, the anode 26 b has thehydrophobic diffusion layer having low porosity. The oxygen-containinggas flows through the first unit cell 12 before the oxygen-containinggas is supplied to the cathode 24 b of the second assembly 20. Thus, theoxygen-containing gas supplied to the cathode 24 b contains waterproduced in the first unit cell 12, i.e., the humidity of theoxygen-containing gas supplied to the cathode 24 b is high. Therefore,the cathode 24 b has the hydrophilic diffusion layer having highporosity so that water can move toward the anode 26 b smoothly.

As described above, in the first embodiment, for example, the first unitcell 12 and the second unit cell 14 are juxtaposed such that theoxygen-containing gas flow passage 32 extends serially from the firstunit cell 12 to the second unit cell 14. In the cell assembly 10, theflow rate of the oxygen-containing gas supplied to the first unit cell12 provided on the upstream side is determined taking the flow rate ofthe oxygen-containing gas supplied to the second unit cell 14 providedon the downstream side into account, so that the sufficient flow rate ofthe oxygen-containing gas required for reaction in the second unit cell14 can be supplied to the second unit cell 14. Thus, the flow rate ofthe oxygen-containing gas supplied into the cell assembly 10 is high.

Therefore, water condensation in the oxygen-containing gas flow passage32 can be prevented, and the humidity is uniform in the first and secondunit cells 12, 14. Further, the current density distribution is uniformin the first and unit cells 12, 14, and thus, the concentrationoverpotential can be reduced. Since the oxygen-containing gas issupplied into the cell assembly 10 at a high speed, the water producedin power generation can be discharged from the first and second unitcells 12, 14 efficiently.

In particular, the first oxygen-containing gas passage 38 is provided onthe upper side of the first assembly 18, and the secondoxygen-containing gas passage 42 is provided on the lower side of thesecond assembly 20. Therefore, the water produced in the first unit cellis reliably discharged from the first unit cell 12 to the second unitcell 14 by the gravity, and then, discharged from the second unit cell14. The excessive water from the first assembly 18 is dischargeddownwardly into the second oxygen-containing gas passage 42 at aposition below the first assembly 18 by the gravity. Thus, with thesimple structure, it is possible to prevent the condensed water frombeing trapped in the first and second assemblies 18, 20.

The oxygen-containing gas flow passage 32 extending through the firstand second unit cells 12, 14 is a long passage. The pressure loss islarge, and thus, the oxygen-containing gas is distributed in the firstand second unit cells 12, 14 efficiently, and the water produced in thefirst and second unit cells 12, 14 is discharged smoothly. The fuel gasflow passage 34 extends serially through the juxtaposed second and firstunit cells 14, 12 such that the fuel gas flows from the second unit cell14 to the first unit cell 12. Thus, the same advantage as with theoxygen-containing gas flow passage 32 can be obtained.

In the cell assembly 10, the first and second unit cells 12, 14 arejuxtaposed such that electrode surfaces of the first unit cell 12 andelectrode surfaces of the second unit cells 14 are aligned in parallelwith each other. Thus, the first unit cell 12 and the second unit cell14 can be handled independently. For example, only the performance testof the fist unit cell 12 can be carried out easily and accurately.

In the first unit cell 12, the low humidified oxygen-containing gas orthe non-humidified oxygen-containing gas flows through the firstoxygen-containing gas passage 38 in the direction indicated by the arrowA, and the fuel gas having a relatively high humidity flows through thesecond fuel gas passage 48 in the direction indicated by the arrow B.Thus, the water in the second fuel gas passage 48 moves from the anode26 a having the hydrophilic diffusion layer of high porosity to thesolid polymer electrolyte membrane 22 a. Therefore, it is possible toreliably prevent the solid polymer electrolyte membrane 22 a from beingdried. Even if the low humidified oxygen-containing gas or thenon-humidified oxygen-containing gas is supplied to the cell assembly10, the desired wet state of the solid polymer electrolyte membrane 22 acan be maintained.

In the second unit cell 14, the oxygen-containing gas of high humidity,containing water produced in power generation flows through the secondoxygen-containing gas passage 42 in the direction indicated by the arrowA, and the low humidified fuel gas or the non-humidified fuel gas flowsthrough the first fuel gas passage 44 in the direction indicated by thearrow B. Thus, the water in the oxygen-containing gas passage 42 movesfrom the cathode 24 b having the hydrophilic diffusion layer of highporosity to the solid polymer electrolyte membrane 22 b. Therefore, itis possible to prevent the solid polymer electrolyte membrane 22 b frombeing dried. Even if the low humidified gas or the non-humidified gas issupplied to the cell assembly 10, the desired wet state of the solidpolymer electrolyte membrane 22 b is maintained.

Next, FIG. 3 shows change in humidity of the first and second assemblies18, 20, the first and second oxygen-containing gas passage 38, 42, andthe first and second fuel gas passage 44, 48 in the first and secondunit cells 12, 14.

In the first unit cell 12, the first assembly 18 is humidified by thefuel gas having high relative humidity flowing through the second fuelgas passage 48. In the second unit cell 14, the second assembly 20 ishumidified by the oxygen-containing gas having high humidity flowingthrough the second oxygen-containing gas passage 42.

Thus, it may not be necessary to humidify the oxygen-containing gas andthe fuel gas in supplying the oxygen-containing gas and the fuel gas tothe cell assembly 10. It is possible to maintain the desired humidity ofthe first and second assemblies 18, 20, and improve the power generationperformance of the first and second unit cells 12, 14.

FIG. 4 shows change in humidity of the first and second unit cells 12,14. In the second unit cell 14, the humidity is high due to the waterproduced in the power generation. The second unit cell 14 is heated, andthe relative humidity of the oxygen-containing gas is lowered (see FIGS.3 and 4). Thus, water does not condense in the second unit cell 14. Thecurrent density distribution is uniform, and the concentrationoverpotential can be reduced.

Further, in the first embodiment, the connection passage member 16 isinterposed between the first and second unit cells 12, 14. Thus, thecell assembly 10 is compact as a whole. The cell assembly 10 can behandled easily, and installed at various positions easily and suitably.

FIG. 5 is a schematic view showing main components of a solid polymercell assembly 80 according to a second embodiment of the presentinvention. The constituent elements that are identical to those of thecell assembly 10 according to the first embodiment are labeled with thesame reference numeral, and description thereof will be omitted.

The cell assembly 80 includes a first fuel cell stack 82 formed bystacking a plurality of, e.g., three first unit cells 12, and a secondfuel cell stack 84 formed by stacking a plurality of, e.g., three secondunit cells 14, and a connection passage member 16 interposed between thefirst fuel cell stack 82 and the second fuel cell stack 84. The firstfuel cell stack 82 and the second fuel cell stack 84 are juxtaposedtogether.

The connection passage member 16 may be formed by a single component.Alternatively, the connection passage member 16 may be formed bystacking three components. The first and second fuel cell stacks 82, 84include manifold members 86, 88 for supplying/discharging theoxygen-containing gas, the fuel gas, and the coolant to/from the firstand second unit cells 12, 14, respectively.

As described above, in the second embodiment, a plurality of the firstand second unit cells 12, 14 are stacked together to form the first andsecond fuel cell stacks 82, 84, respectively for achieving the highoutput easily. Further, in the structure in which the oxygen-containinggas can be supplied externally to the connection passage member 16, itis possible to effectively reduce the flow rate of the oxygen-containinggas supplied to the first fuel cell stack 82.

INDUSTRIAL APPLICABILITY

According to the present invention, the flow rate of the reactant gassupplied to the unit cell on the upstream side is high since the flowrate of the reactant gas supplied to the unit cell on the downstreamside is taken into account. Thus, it is possible to prevent the watercondensation in the reactant gas flow passage, and the humidity isuniform in each of the unit cells. Accordingly, the current densitydistribution is uniform in each of the unit cells, and the concentrationoverpotential can be reduced.

The reactant gas flows at a high speed so that the water produced inpower generation can be discharged from the unit cells efficiently.Further, a plurality of the unit cells are juxtaposed such thatelectrode surfaces of the unit cells are aligned in parallel with eachother. Thus, the unit cells can be handled independently. Therefore, forexample, the performance test can be performed individually for each ofthe unit cells easily and reliably.

1. A solid polymer cell assembly comprising a cell assembly formed byjuxtaposing a plurality of unit cells such that electrode surfaces ofsaid unit cells are aligned in parallel with each other, said unit celleach having an assembly including an anode, a cathode, and a solidpolymer electrolyte membrane interposed between said anode and saidcathode, wherein said unit cells includes an upstream unit cell providedon an upstream side in a flow direction of a reactant gas including atleast one of an oxygen-containing gas and a fuel gas, and a downstreamunit cell provided on a downstream side in the flow direction, andwherein said unit cells include an upstream unit cell provided on theupstream side in a flow direction of the oxygen-containing gas, and adownstream unit cell provided on the downstream side in the flowdirection of the oxygen-containing gas; wherein a coolant flow passageis provided such that a coolant flows serially from said upstream unitcell provided on the upstream side in the flow direction of theoxygen-containing gas to said downstream unit cell provided on thedownstream side in the flow direction of the oxygen-containing gas sothat temperature of said downstream unit cell provided on the downstreamside in the flow direction of the oxygen-containing gas is kept higherthan temperature of said upstream unit cell provided on the upstreamside in the flow direction of the oxygen-containing gas; and at leastpart of a reactant gas flow passage for said reactant gas extendsserially from a passage formed on an upper side of the assembly of saidupstream unit cell to a passage formed on a lower side of the assemblyof said downstream unit cell, wherein said reactant gas flow passageincludes a fuel gas flow passage and an oxygen-containing gas flowpassage, and the oxygen-containing gas and the fuel gas flows in acounterflow manner in the oxygen-containing gas flow passage and thefuel gas flow passage along both surfaces of the assemblies of said unitcells.
 2. A cell assembly according to claim 1, wherein structure ofsaid upstream unit cell is different from structure of said downstreamunit cell.
 3. A cell assembly according to claim 2, the assembly of saidupstream unit cell and the assembly of said downstream unit cell havethe same power generation performance when the assembly of said upstreamunit cell is operated at a low temperature in comparison with theassembly of said downstream unit cell.
 4. A cell assembly according toclaim 2, wherein said cathode of the assembly of said upstream unit cellhas a hydrophobic diffusion layer having low porosity, and said anode ofthe assembly of said upstream unit cell has a hydrophilic diffusionlayer having high porosity; and said hydrophobic diffusion layer havinglow porosity is provided on the upper side, and said hydrophilicdiffusion layer having high porosity is provided on the lower side.
 5. Acell assembly according to claim 2, wherein said anode of the assemblyof said downstream unit cell has a hydrophobic diffusion layer havinglow porosity, and said cathode of the assembly of said downstream unitcell has a hydrophilic diffusion layer having high porosity; and saidhydrophobic diffusion layer having low porosity is provided on the upperside, and said hydrophilic diffusion layer having high porosity isprovided on the lower side.
 6. A solid polymer cell assembly comprisinga cell assembly formed by juxtaposing a plurality of unit cells suchthat electrode surfaces of said unit cells are aligned in parallel witheach other, said unit cells each having an assembly including an anode,a cathode, and a solid polymer electrolyte membrane interposed betweensaid anode and said cathode, wherein said unit cells includes anupstream unit cell provided on an upstream side in a flow direction of areactant gas including at least one of an oxygen-containing gas and afuel gas, and a downstream unit cell provided on a downstream side inthe flow direction; wherein at least part of a reactant gas flow passagefor said reactant gas extends serially from a passage formed on an upperside of the assembly of said upstream unit cell to a passage formed on alower side of the assembly of said downstream unit cell; wherein aconnection passage member is provided between said juxtaposed unitcells; and a reactant gas connection passage and a coolant connectionpassage are formed in said connection passage member for seriallysupplying the reactant gas and the coolant.